Human skin consists of two layers, the dermis and the epidermis. The epidermis, which is the uppermost of the two skin layers, encompasses many different cell types, including melanocytes and keratinocytes. Melanocytes are specialized cells in the basal layer of the epidermis which synthesize melanin; the melanin is then packaged into melanosomes and then transported into keratinocytes. Exposure of skin to the sun results in vitamin D synthesis, sunburn (erythema), and tanning, the skin's major form of endogenous protection against subsequent skin damage from ultraviolet (UV) irradiation. Various morphologic and enzymatic changes occur at the cellular level in epidermal melanocytes in response to UV irradiation. Melanin, which is increased in "tanned" skin, serves as a filter with absorbance within the UV range and provides photoprotection for the individual.
The peak action spectrum for erythema is in the UV-B range, 290-305 nm. UV-B rays are absorbed by proteins and nucleic acids of the epidermis, causing the production of thymine dimers, which are known to be formed by UV irradiation of nuclear DNA and to be excised from the DNA strand by the action of highly specific enzymes, including endonucleases. If not removed, these dimers can stall DNA replication forks generating regions of single-stranded DNA. Failure to remove thymine dimers and other forms of DNA damage in the genome may lead to somatic mutations resulting in carcinogenesis.
In bacteria it is known that single-stranded DNA released as fragments during the course of DNA repair or exposed at stalled replication forks can interact with nuclear proteins which then regulate the expression of specific genes in the DNA as part of the organism's SOS response to UV damage. The tanning response of skin might reasonably be considered part of the analogous SOS response in mammalian skin. The precise stimulus for UV-induced tanning, however, remains unknown.
UV irradiation is successfully used in phototherapy and photochemotherapy for certain dermatological conditions. For example, psoriasis is a common dermatologic disease affecting 1 to 2 percent of the population. Psoriasis can be treated with UV-B irradiation, either alone or in conjunction with agents such as coal tar or anthralin, or with UV-A irradiation in combination with psoralens (PUVA therapy). Other diseases which respond to UV irradiation treatment include atopic dermatitis and vitiligo. Despite the benefits of phototherapy and photochemotherapy, these treatments carry the same risks as chronic exposure to sun, including wrinkling, "photoaging," and skin cancer.